I don't have a K-12 teaching qualification. I'm considering doing a distance middle school qualification through Australia or an MA in Distance Ed through Canada. I've also considered doing DELTA. Any advice on which one would make me more employable in the German or wider European job market?

I'm not too worried about salary in the short term. I'll be there with my partner who may be stationed in Germany (DOD).

If all 16 of your years are in Japan, be sure to get whatever future quals you get in Germany or in Europe. The differences between teaching Asian students as versus German ones are profound, and many employers may be concerned about whether you'll be able to make the transition effectively.

When I was on a hiring committee in the Netherlands, on the German border, we were very wary of applicants with experience 'only' in Asia.
That is NOT to say you won't be absolutely fine; but it can help you in the job application process if you show some awareness of the differences (they may be only 'perceived' differences to some extent, but they do come into play in the hiring process in many cases).

Here's a thread that discusses this notion from the perspective of several teachers who've done both. The second page continues to give current info regarding the relevant differences (it doesn't actually shift focus to other parts of the world as suggested):

On the other topic, in your case I'd go for a DELTA in Germany. It would give you supervised experience teaching German students and you can have it done in a relatively short time. An MA in distance learning...I'm not sure of the relevance of that to most employers. And as others have mentioned, the K-12 positions are very limited; usually filled by well-qualified locals.

The first five years of my career was spent teaching English to new Canadians and international students in Vancouver - writing prep, business English, TOEFL prep. The schools and the learners expected results.

I'm well aware of teacher-culture shock. In Vancouver ESL schools we saw ESL teachers returning from Japan get overwhelmed in our general and business English programs.

DELTA is attractive, but is it useful in other job markets? Certainly a great way to get oriented to Germany and build connections. But beyond that?

DELTA, being the step up from CELTA, but shorter than an MA, is often required for people moving into DOS positions, or teacher training. Like a CELTA, it's recognized in the whole region, not just in Germany.

I suggested it because it would give you supervised teaching practice with European students (this will look good on your CV after the Asian and Canadian experience), it's higher than an entry-level thing, and it's relatively short.